1. Field of the Invention
This invention relates to processes for preparing 2,4-dihydroxypyridine and 2,4-dihydroxy-3-nitropyridine which compounds are intermediates that are useful in preparing adenosine compounds and analogs thereof which are useful in treating hypertension and myocardial ischemia, as cardioprotective agents which ameliorate ischemic injury or myocardial infarct size consequent to myocardial ischemia, and as antilipolytic agents which reduce plasma lipid levels, serum triglyceride levels, and plasma cholesterol levels.
Hypertension
Hypertension, a condition of elevated blood pressure, affects a substantial number of the human population. Consequences of persistent hypertension include vascular damage to the ocular, renal, cardiac and cerebral systems, and the risk of these complications increases as blood pressure increases. Basic factors controlling blood pressure are cardiac output and peripheral vascular resistance, with the latter being the predominant common mechanism which is controlled by various influences. The sympathetic nervous system regulates peripheral vascular resistance through direct effects on alpha- and beta-adrenergic receptors as well as through indirect effects on renin release. Drug therapy is aimed at specific components of these blood pressure regulatory systems, with different mechanisms of action defining the several drug classes including diuretics, beta-adrenergic receptor antagonists (beta-blockers), angiotensin-converting enzyme (ACE) inhibitors, and calcium channel antagonists.
Thiazide-type diuretics are used in hypertension to reduce peripheral vascular resistance through their effects on sodium and water excretion. This class of drugs includes hydrochlorothiazide, chlorothiazide, methyclothiazide, and cyclothiazide, as well as related agents indapamide, metolazone, and chlorthalidone. Although the beta-blocker mechanism of action was once believed to be blockade of the beta.sub.1 -adrenergic receptor subtype in the heart to reduce heart rate and cardiac output, more recent beta-blockers with intrinsic sympathomimetic activity (ISA), including pindolol, acebutolol, penbutolol, and carteolol, are as effective as non-ISA beta-blockers, causing less reduction in heart rate and cardiac output. Other postulated mechanisms for these drugs include inhibition of renin release, a central effect, and an effect at pre-synaptic beta-adrenergic receptors resulting in inhibition of norepinephrine release. Cardioselective beta-blockers metoprolol (Lopressor-Geigy), acebutolol (Sectral-Wyeth), and atenolol (Tenormin-ICI), at low doses, have a greater effect on beta.sub.1 -adrenergic receptors than on beta.sub.2 -adrenergic receptor subtypes located in the bronchi and blood vessels. Nonselective beta-blockers act on both beta-adrenergic receptor subtypes and include propranolol (Inderal-Ayerst), timolol (Blocadren-Merck), nadolol (Corgard-Squibb), pindolol (Visken-Sandoz), penbutolol (Levatol-Hoechst-Roussel), and carteolol (Cartrol-Abbott). Adverse effects of beta-blockers include asymptomatic bradycardia exacerbation of congestive heart failure, gastrointestinal disturbances, increased airway resistance, masked symptoms of hypoglycemia, and depression. They may cause elevation of serum triglycerides and may lower high-density lipoprotein cholesterol.
ACE inhibitors prevent the formation of angiotensin II and inhibit breakdown of bradykinin. Angiotensin II is a potent vasoconstrictor and also stimulates the secretion of aldosterone. By producing blockade of the renin-angiotensin-aldosterone system, these agents decrease peripheral vascular resistance, as well as sodium and water retention. In addition, ACE inhibitors increase levels of bradykinin and prostaglandins, endogenous vasodilators. Captopril (Capoten-Squibb) and Enalapril (Vasotec-Merck) are the leading ACE inhibitors. Adverse effects of the ACE inhibitors include rash, taste disturbance, proteinuria and neutropenia.
The calcium channel antagonists reduce the influx of calcium into vascular smooth muscle cells and produce systemic vasodilation, resulting in their antihypertensive effect. Other effects of calcium channel antagonists include interference with action of angiotensin II and alpha.sub.2 -adrenergic receptor blockade, which may add to their antihypertensive effects. Calcium channel antagonists do not have the adverse metabolic and pharmacological effects of thiazides or beta-blockers and may therefore be useful in patients with diabetes, peripheral vascular disease, or chronic obstructive pulmonary disease. Two calcium channel antagonists, Verapamil and diltiazem, have serious adverse cardiovascular effects on atrioventricular cardiac conduction in patients with preexisting conduction abnormalities, and they may worsen bradycardia, heart block, and congestive heart failure. Other minor adverse effects of calcium channel antagonists include peripheral edema, dizziness, light-headedness, headache, nausea, and flushing, especially with nifedipine and nicardipine.
Many other agents are available to treat essential hypertension. These agents include prazosin and terazocin, alpha.sub.1 -adrenergic receptor antagonists whose antihypertensive effects are due to resultant arterial vasodilation; clonidine, an alpha.sub.2 -adrenergic agonist which acts centrally as well as peripherally at inhibitory alpha.sub.2 -adrenergic receptors, decreasing sympathetic response. Other centrally acting agents include methyldopa, guanabeiz, and guanfacine; reserpine, which acts by depleting stores of catecholamines; guanadrel, a peripheral adrenergic antagonist similar to guanethidine with a shorter duration of action; and direct-acting vasodilators such as hydralazine and minoxidil. These agents, although effective produce noticeable symptomatic side effects, including reflex sympathetic stimulation and fluid retention, orthostatic hypotension, and impotence.
Many antihypertensive agents activate compensatory pressor mechanisms, such as increased renin release, elevated aldosterone secretion and increased sympathetic vasoconstrictor tone, which are designed to return arterial pressure to pretreatment levels, and which can lead to salt and water retention, edema and ultimately to tolerance to the antihypertensive actions of the agent. Furthermore, due to the wide variety of side effects experienced with the present complement of antihypertensive drugs and the problems experienced therewith by special populations of hypertensive patients, including the elderly, blacks, and patients with chronic obstructive pulmonary disease, diabetes, or peripheral vascular diseases, there is a need for additional classes of drugs to treat hypertension.
Ischemia
Myocardial ischemia is the result of an imbalance of myocardial oxygen supply and demand and includes exertional and vasospastic myocardial dysfunction. Exertional ischemia is generally ascribed to the presence of critical atherosclerotic stenosis involving large coronary arteries resulting in a reduction in subendocardial flow. Vasospastic ischemia is associated with a spasm of focal variety, whose onset is not associated with exertion or stress. The spasm is better defined as an abrupt increase in vascular tone. Mechanisms for vasospastic ischemia include: (i) Increased vascular tone at the site of stenosis due to increased catecholamine release: (ii) Transient intraluminal plugging and (iii) Release of vasoactive substances formed by platelets at the site of endothelial lesions.
The coronary circulation is unique since it perfuses the organ which generates the perfusion pressure for the entire circulation. Thus, interventions which alter the state of the peripheral circulation and contractility will have a profound effect on coronary circulation. The regulatory component of the coronary vasculature is the small coronary arterioles which can greatly alter their internal diameter. The alteration of the internal radius is the result of either intrinsic contraction of vascular smooth muscle (autoregulation) or extravascular compression due to ventricular contraction. The net effect of therapies on the ischemic problem involves a complex interaction of opposing factors which determine the oxygen supply and demand.
Cardioprotection and Prevention of Ischemic Injury
The development of new therapeutic agents capable of limiting the extent of myocardial injury, i.e., the extent of myocardial infarction, following acute myocardial ischemia is a major concern of modern cardiology.
The advent of thrombolytic (clot dissolving) therapy during the last decade demonstrates that early intervention during heart attack can result in significant reduction of damage to myocardial tissue. Large clinical trials have since documented that thrombolytic therapy decreases the risk of developing disturbances in the heartbeat and also maintains the ability of the heart to function as a pump. This preservation of normal heart function has been shown to reduce long-term mortality following infarction.
There has also been interest in the development of therapies capable of providing additional myocardial protection which could be administered in conjunction with thrombolytic therapy, or alone, since retrospective epidemiological studies have shown that mortality during the first few years following infarction appears to be related to original infarct size.
In preclinical studies of infarction, conducted in a variety of animal models, many types of pharmacological agents such as calcium channel blockers, prostacyclin analogs, and agents capable of inhibiting certain metabolic pathways have been shown to be capable of reducing ischemic injury in several animal species.
Recent studies have demonstrated that exposure of the myocardium to brief periods of ischemia (interruption of blood flow to the heart) followed by reperfusion (restoration of blood flow) is able to protect the heart from the subsequent ischemic injury that would otherwise result from subsequent exposure to a longer period of ischemia. This phenomenon has been termed myocardial preconditioning and is believed to be partially attributable to the release of adenosine during the preconditioning period.
Other studies have shown that adenosine and adenosine agonists reduce the extent of tissue damage that is observed following the interruption of blood flow to the heart in a variety of models of ischemic injury in several species (see, for example, Toombs. C. et al., "Myocardial protective effects of adenosine. Infarct size reduction with pretreatment and continued receptor stimulation during ischemia.", Circulation 86, 986-994 (1992): Thornton, J. et al., "Intravenous pretreatment with A.sub.1 -selective adenosine analogs protects the heart against infarction.", Circulation 85, 659-665 (1992); and Downey, J., "Ischemic preconditioning--nature's own cardioprotective intervention.", Trends Cardiovasc. Med. 2(5), 170-176 (1992)).
The processes of the present invention prepares intermediates which are useful in preparing compounds which mimic myocardial preconditioning, thereby ameliorating ischemic injury or producing a reduction in the size of myocardial infarct consequent to myocardial ischemia and are useful as cardioprotective agents.
Antilipolvsis
Hyperlipidemia and hypercholesterolemia are known to be two of the prime risk factors for atherosclerosis and coronary heart disease, the leading cause of death and disability in Western countries. Although the etiology of atherosclerosis is multifactorial, the development of atherosclerosis and conditions including coronary artery disease, peripheral vascular disease and cerbrovascular disease resulting from restricted blood flow, are associated with abnormalities in serum cholesterol and lipid levels. The etiology of hypercholesterolemia and hyperlipidemia is primarily genetic, although factors such as dietary intake of saturated fats and cholesterol may contribute.
The antilipolytic activity of adenosine and adenosine analogues arise from the activation of the A.sub.1 receptor subtype (Lohse, M. J., et al., Recent Advances in Receptor Chemistry, Melchiorre, C. and Gianella, Eds, Elsevier Science Publishers B. V. Amsterdam, 1988, 107-121). Stimulation of this receptor subtype lowers the intracellular cyclic AMP concentration in adipocytes. Cyclic AMP is a necessary co-factor for the enzyme lipoprotein lipase which hydrolytically cleaves triglycerides to free fatty acids and glycerol in adipocytes (Egan, J. J., et al., Proc. Natl. Acad. Sci. 1992 (89), 8357-8541). Accordingly, reduction of intracellular cyclic AMP concentration in adipocytes reduces lipoprotein lipase activity and, therefore, the hydrolysis of triglycerides.
Elevated blood pressure and plasma lipids, including triglycerides, are two will accepted risk factors associated with mortality resulting from cardiovascular disease.
For the diabetic patient, where the likelihood of mortality from cardiovascular disease is substantially greater, the risk associated with these factors is further magnified (Bierman, E. L., Arteriosclerosis and Thrombosis 1992 (12), 647-656). Additionally, data suggest that excessive lipolysis is characteristic of non-insulin dependent diabetes and possibly contributes to insulin resistance and hyperglycemia (Swislocki, A. L., Horm. Metab. Res. 1993 (25), 90-95).
The processes of the present invention prepares intermediates which are useful in preparing compounds which are antihypertensive and antilipolytic agents and useful in the treatment and amelioration of both vascular and metabolic risk factors.
Adenosine Compounds And Their Activity
Adenosine has a wide variety of physiological and pharmacological action including a marked alteration of cardiovascular and renal function. In animals and man, intravenous injection of the adenosine nucleotide causes hypotension.
The physiological and pharmacological actions of adenosine are mediated through specific receptors located on cell surfaces. Two adenosine receptor subtypes designated as A.sub.1 and A.sub.2 receptors, have been identified. The A.sub.1 receptor inhibits the formation of cAMP by suppressing the activity of adenylate cyclase, while stimulation of A.sub.2 receptors increases adenylate cyclase activity and intracellular cAMP. Each receptor appears to mediate specific actions of adenosine in different tissues: for example, the vascular actions of adenosine appears to be mediated through stimulation of A.sub.2 receptors, which is supported by the positive correlation between cAMP generation and vasorelaxation in adenosine-treated isolated vascular smooth muscle; while stimulation of the cardiac A.sub.1 receptors reduces cAMP generation in the heart which contributes to negative dromotropic, inotropic and chroniotropic cardiac effects. Consequently, unlike most vasodilators, adenosine administration does not produce a reflex tachycardia.
Adenosine also exerts a marked influence on renal function. Intrarenal infusion of adenosine causes a transient fall in renal blood flow and an increase in renal vascular resistance. With continued infusion of adenosine, renal blood flow returns to control levels and renal vascular resistance is reduced. The initial renal vasoconstrictor responses to adenosine are not due to direct vasoconstrictor actions of the nucleotide, but involve an interaction between adenosine and the renin-angiotensin system.
Adenosine is widely regarded as the primary physiological mediator of reactive hyperemia and autoregulation of the coronary bed in response to myocardial ischemia. It has been reported that the coronary endothelium possesses adenosine A.sub.2 receptors linked to adenylate cyclase, which are activated in parallel with increases in coronary flow and that cardiomyocyte receptors are predominantly of the adenosine A.sub.1 subtype and associated with bradycardia. Accordingly, adenosine offers a unique mechanism of ischemic therapy.
Cardiovascular responses to adenosine are short-lived due to the rapid uptake and metabolism of the endogenous nucleotide. In contrast, the adenosine analogs are more resistant to metabolic degradation and are reported to elicit sustained alterations in arterial pressure and heart rate.
Several potent metabolically-stable analogs of adenosine have been synthesized which demonstrate varying degrees of selectivity for the two receptor subtypes. Adenosine agonists have generally shown greater selectivity for A.sub.1 receptors as compared to A.sub.2 receptors. Cyclopentyladenosine (CPA) and R-phenylisopropyl-adenosine (R-PIA) are standard adenosine agonists which show marked selectivity for the A.sub.1 receptor (A.sub.2 /A.sub.1 ratio=780 and 106, respectively). In contrast, N-5'-ethyl-carboxamido adenosine (NECA) is a potent A.sub.2 receptor agonist (Ki-12 nM) but has equal affinity for the A.sub.1 receptor(Ki-6.3 nM; A.sub.2 /A.sub.1 ratio=1.87). Until recently, CV-1808 was the most selective A.sub.2 agonist available (A.sub.2 /A.sub.1 =0.19), even though the compound was 10-fold less potent than NECA in its affinity for the A.sub.2 receptor. In recent developments, newer compounds have been disclosed which are very potent and selective A.sub.2 agonists (Ki=3-8 nM for A.sub.1 ; A.sub.2 /A.sub.1 ratio=0.027-0.042).
Various N6-aryl and N6-heteroarylalkyl substituted adenosines, and substituted-(2-amino and 2-hydroxy)adenosines, have been reported in the literature as possessing varied pharmacological activity, including cardiac and circulatory activity. See, for example, British Patent Specification 1,123,245, German Offen. 2,136,624, German Off 2,059,922, German Offen. 2,514,284, South African Patent No. 67/7630, U.S. Pat. No. 4,501,735, EP Publication No. 0139358 (disclosing N6-[geminal diaryl substituted alkyl]adenosines), EP Patent Application Ser. No. 88106818.3 (disclosing that N6-heterocyclic-substituted adenosine derivatives exhibit cardiac vasodilatory activity), German Offen. 2,131,938 (disclosing aryl and heteroaryl alkyl hydrazinyl adenosine derivatives), German Offen. 2,151,013 (disclosing N6-aryl and heteroaryl substituted adenosines), German Offen. 2,205,002 (disclosing adenosines with N6-substituents comprising bridged ring structures linking the N6-nitrogen to substituents including thienyl) and South African Patent No. 68/5477 (disclosing N6-indolyl substituted-2-hydroxy adenosines).
U.S. Pat. No. 4,954,504 and EP Publication No. 0267878 disclose generically that carbocyclic ribose analogues of adenosine, and pharmaceutically acceptable esters thereof, substituted in the 2- and/or N6-positions by aryl lower alkyl groups including thienyl, tetrahydropyranyl, tetrahydrothiopyranyl, and bicyclic benzo fused 5- or 6-membered saturated heterocyclic lower alkyl derivatives exhibit adenosine receptor agonist properties. Adenosine analogues having thienyl-type substituents are described in EP Publication No. 0277917 (disclosing N6-substituted-2-heteroarylalkylamino substituted adenosines including 2-[(2-[thien-2-yl]ethyl)amino] substituted adenosine), German Offen. 2,139,107 (disclosing N6-[benzothienylmethyl]-adenosine). PCT WO 85/04882 (disclosing that N6-heterocyclicalkyl-substituted adenosine derivatives, including N6-[2-(2-thienyl)ethyl]amino-9-(D-ribofuranosyl)-9H-purine, exhibit cardiovascular vasodilatory activity and that N6-chiral substituents exhibit enhanced activity), EP Published Application No. 0232813 (disclosing that N6-(1-substituted thienyl)cyclopropylmethyl substituted adenosines exhibit cardiovascular activity), U.S. Pat. No. 4,683,223 (disclosing that N6-benzothiopyranyl substituted adenosines exhibit antihypertensive properties), PCT WO 88/03147 and WO 88/03148 (disclosing that N6-[2-aryl-2-(thien-2-yl)]ethyl substituted adenosines exhibit antihypertensive properties), U.S. Pat. Nos. 4,636,493 and 4,600,707 (disclosing that N6-benzothienylethyl substituted adenosines exhibit antihypertensive properties).
Adenosine-5'-carboxylic acid amides are disclosed as having utility as anti-hypertensive and anti-anginal agents in U.S. Pat. No. 3,914,415, while U.S. Pat. No. 4,738,954 discloses that N6-substituted aryl and arylalkyl-adenosine 5'-ethyl carboxamides exhibit various cardiac and antihypertensive properties.
N.sup.6 -alkyl-2'-O-alkyl adenosines are disclosed in EP Publication No. 0,378,518 and UK Patent Application 2,226,027 as having antihypertensive activity. N.sup.6 -alkyl-2',3'-di-O-alkyl adenosines are also reported to have utility as antihypertensive agents, U.S. Pat. No. 4,843,066.
Adenosine-5'-(N-substituted)carboxamides and carboxylate esters and N1-oxides thereof are reported to be coronary vasodilators, Stein. et al., J. Med. Chem,. 1980, 23, 313-319 and J. Med. Chem. 19 (10), 1180 (1976). Adenosine-5'-carboxamides and N1-oxides thereof are also reported as small animal poisons in U.S. Pat. No. 4,167,565.
The antilipolytic activity of adenosine is described by Dole. V. P., J. Biol. Chem. 236 (12), 3125-3130(1961). Inhibition of lipolysis by (R)-N.sup.6 phenylisopropyl adenosine is disclosed by Westermann, E., et al., Adipose Tissue, Regulation and Metabolic Functions, Jeanrenaud, B. and Hepp, D. Eds., George Thieme, Stuttgart, 47-54 (1970). N.sup.6 -mono- and disubstituted adenosine analogues are disclosed as having antilipolytic, antihypercholesterolemic, and antihyperlipemic activity in U.S. Pat. Nos. 3,787,391, 3,817,981, 3,838,147, 3,840,521, 3,835,035, 3,851,056, 3,880,829, 3,929,763, 3,929,764, 3,988,317, and 5,032,583.
It is believed that the reported toxicity, CNS properties and heart rate elevation associated with adenosine analogues have contributed to the difficulties preventing the development of a commercial adenosine analog antihypertensive/antiischemic agent.
U.S. patent application Ser. Nos. 08,484,811 and 08,316/761, which claim benefit of published PCT Application PCT/US91/06990, disclose a class of metabolically stable adenosine agonists, and derivatives thereof, possessing unexpectedly desirable pharmacological properties. i.e., anti-hypertensive, cardioprotective, anti-ischemic, and antilipolytic agents having a unique therapeutic profile.
2. Reported Developments